Methinks a little prejudice took over here. Let's look at what might be typical cut-out dimensions for a cat flap:
Inches
6 3/8 x 7 5/8
mm
162 x 194
How many digits did you say? I count two lots of three both cases
I honestly started in mm because that was the first figures available - then moved to inches.
Although my measures were "something and a half" by "something" your above example is still interesting - since a big "6" and something x a big 7 and something is easier to store in the old brain than one hundred and sometysomething (although I can predict a reply that goes along the route 'root' ;-) of shifting decimal points).
P.S. I initially went for mm because I usually prefer mm for small distances - but in this case 'mm' was too small for me as I'd moved into the "next step up" measure, which for me is inches.
